1. Field of the Invention
The present invention relates generally to battery-powered devices and, more particularly, to measuring battery depletion in battery-powered implantable medical devices.
2. Background Art
Implantable medical devices, such as implantable cardiac devices (ICDs) (e.g., pacemakers, cardioverters, and defibrillators), implantable neurostimulator devices, blood glucose monitoring/delivery devices, etc., are typically battery powered. The state of battery depletion can be monitored or estimated in order to determine the elective replacement (ER) and end-of-life (EOL) points, which enable a physician to schedule appropriate device replacement.
One approach for determining battery depletion is to monitor the battery terminal voltage during the lifetime of the battery in the implantable medical device. Typically the battery terminal voltage decreases as the battery is depleted until a threshold voltage is reached, indicating that the battery needs to be replaced. A problem with this approach is some battery chemistries, such as carbon monofloride, have terminal characteristics, including terminal voltage and source impedance, which do not change according to the battery's state of depletion. Additionally, monitoring changes in the battery's terminal voltage might not be an accurate indicator of the battery's state of depletion because the change in terminal voltage between a new battery and a battery at EOL is relatively small, typically 0.2 V.
Another approach for determining battery depletion is to measure the total integrated current (Ampere-hours) drawn from the battery during its lifetime in the implantable medical device. Measuring the total integrated current to determine battery depletion is useful for battery chemistries, such as carbon monofloride, which have terminal characteristics that do not change according to the battery's state of depletion. Additionally, by recording the total integrated current on a daily, weekly, or other periodic basis, a measure of the rate of battery depletion (which may increase or decrease over time depending upon the programmed patient therapy and physiological needs) may also be determined. The measure of the rate of battery depletion is clinically useful in predicting when the battery will reach the ER or EOL points.
A conventional current integrating circuit for measuring total integrated current drawn from a battery in an implantable medical device is a voltage-controlled oscillator driven by a resistor coupled between the battery and a battery reservoir capacitor. A problem with this approach is it operates over a limited dynamic range of current drawn from the battery. For example, a current integrating circuit in an ICD should be capable of integrating lower background current drawn by ICD sensing electronics that is typically less than 10 μA, as well as higher current bursts drawn for burst pacing or high speed telemetry that are typically greater than 1 mA.
A disadvantage of conventional voltage-controlled oscillator current integrating circuits is that they require multiple passive elements (i.e., resistors) in order to accommodate a wide dynamic range of current drawn from the battery. Furthermore, because the voltage drop across passive elements in conventional voltage-controlled oscillator current integrating circuits is dependent on the average current through them, the conventional circuits suffer from reduced accuracy when integrating over a wide dynamic range of current drawn from the battery.
What is needed are a better method and apparatus for determining the state of depletion of a battery in an implantable medical device for a wide dynamic range of current drawn from the battery.